android - 为什么 LiveData 不更新？ (安卓、 Kotlin )

Question

如何将 MutableLiveData 公开为 LiveData？

下面的代码不起作用。

SimpleMixerFragment.kt

private lateinit var binding: MixerSimpleFragmentBinding
private lateinit var viewModel: SimpleMixerViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    // Inflate view and obtain an instance of the binding class
    binding = DataBindingUtil.inflate(
        inflater,
        R.layout.mixer_simple_fragment,
        container,
        false
    )

    viewModel = ViewModelProvider(this).get(SimpleMixerViewModel::class.java)

    binding.simpleMixerViewModel = viewModel

    binding.lifecycleOwner = viewLifecycleOwner

    return binding.root
}

SimpleMixerViewModel.kt

class SimpleMixerViewModel : ViewModel() {

   private val _etDate = MutableLiveData<String>()
   val etDate: LiveData<String>
       get() = _etDate
}

Mixer_simple_fragment.kt

<variable name="simpleMixerViewModel" type="....SimpleMixerViewModel" />

<com.google.android.material.textfield.TextInputEditText
                                android:id="@+id/etDate"                
                                android:text="@{simpleMixerViewModel.etDate}"/>

这可以代替。

SimpleMixerViewModel.kt

class SimpleMixerViewModel : ViewModel() {

   val etDate = MutableLiveData<String>()
}

Mixer_simple_fragment.xml

<variable name="simpleMixerViewModel" type="....SimpleMixerViewModel" />

<com.google.android.material.textfield.TextInputEditText
                                android:id="@+id/etDate"                
                                android:text="@={simpleMixerViewModel.etDate}"/>

Answer 1

@{}注册单向绑定(bind)，这意味着如果您修改 MutableLiveData从代码中，然后它将更改传播到 EditText .
@={}注册双向绑定(bind)，这意味着如果您修改 MutableLiveData或 android:text属性，那么它将更新 MutableLiveData -> android:text或 android:text -> MutableLiveData (以被修改者为准)。

要使用双向绑定(bind)，您必须公开 LiveData 的可变版本，以便绑定(bind)可以将 EditText 的新值写入 MutableLiveData。